This article has a correction. Please see:

Despite sharing similar domain architecture, it is likely that receptor kinases evolved separately in plants and animals. Although plant receptor kinases are most closely related to an invertebrate cytoplasmic serine-threonine kinase, two receptor kinases that participate in brassinosteroid signaling in Arabidopsis thaliana demonstrate tyrosine autophosphorylation. Upon ligand binding to its extracellular domain, the receptor kinase BRASSINOSTEROID INSENSITIVE 1 (BRI1) undergoes a conformational change that activates its kinase domain, resulting in multiple autophosphorylation events and the release of its inhibitor BRI1 KINASE INHIBITOR (BKI1) from the plasma membrane. Jaillais et al. identified a motif in BKI1 that was required for its localization to the plasma membrane and found that BKI1 released from the membrane in brassinosteroid-treated plants was tyrosine-phosphorylated. BKI1 was not tyrosine-phosphorylated in bri1 null mutants, nor was it phosphorylated by a kinase-dead version of BRI1 in vitro. Mutating tyrosine residue 211 of BKI1’s membrane localization domain to phenylalanine (Y211F) prevented both tyrosine phosphorylation of BKI1 and its dissociation from the membrane in response to brassinosteroid stimulation. Overexpression of BKI1Y211F in plants caused a dwarf phenotype similar to that resulting from overexpression of a constitutively membrane-localized version of BKI1, whereas overexpression of BKI1Y211D, which is constitutively cytosolic, gave rise to no abnormal phenotype. The authors determined that a C-terminal domain of BKI1 was required for its interaction with the kinase domain of BRI1 in vitro and that mutation or deletion of the BKI1 C-terminal domain prevented both the dwarf phenotype induced by BKI1 overexpression and coimmunoprecipitation of BKI1 with BRI1. The C-terminal domain of BKI1 bound to a region of BRI1 that is required for the interaction of BRI1 with its coreceptor BAK1, suggesting that BKI1 inhibited BRI1 activity by preventing receptor heterodimerization. Regulation of the subcellular localization of proteins is one of the many effects of tyrosine phosphorylation in animals, and this observation of a similar role for tyrosine phosphorylation in a plant suggests that plant and animal receptor kinase signaling systems, despite having evolved independently, may have converged on a common mechanism for regulation.